In electrocardiographic measuring and monitoring systems, it is desirable to provide a two-dimensional display of time-varying ECG signals on the screen of a cathode ray tube. Typically, the horizontal (X) axis of the display represents time while the vertical (Y) axis represents signal amplitude. It is essential in producing a meaningful display that the horizontal sweeping of the beam across the cathode ray tube screen be synchronized with the ECG signal in a manner similar to that of a triggered oscilloscope.
It is also desirable in such systems to provide an alarm in the event of heart malfunction. Reliable detection of the electrical signal related to heart pumping action is essential both to the implementation of the synchronized display and to the measurement of heart beat rate to monitor the condition of the heart. However, it is extremely difficult to obtain reliable triggering and heartbeat information in the presence of motion artifacts, muscle noise, fluctuations in R-wave amplitude, and electronic heart pacer devices.
In previous systems, it has been a common practice to employ a level comparator to provide a trigger for the sweep and for the rate detector whenever the ECG signal amplitude surpasses a predetermined amplitude level. The R wave is normally of greater amplitude than any other component of the ECG cycle, and therefore is generally used for such triggering and rate detection purposes. However, a major drawback to this scheme is imposed in the presence of electrical and muscle noise on the ECG signal, which may have sufficient amplitude to inadvertently effect triggering of the sweep. Furthermore, devices such as pacers continue to operate and produce a triggered sweep long after a patient has expired. Therefore, it is imperative in monitoring situations such as in intensive care units that only the physiologically-produced signals be recognized.